1. Field of the Invention
This present invention relates to a semiconductor memory device and a manufacturing method thereof, for example, a nonvolatile semiconductor memory device and its manufacturing method.
2. Description of the Related Art
In a semiconductor device, it is important that a boundary between a silicon substrate and a silicon oxide layer thereon is stable. From this reason, a step of a hydrogen annealing is usually used at a manufacturing step of the semiconductor device in order to terminate a dangling bond of silicon that makes an interface state between a silicon substrate and a silicon oxide layer. At the step of the hydrogen annealing, a mixture gas of hydrogen and nitrogen is used to make a bond of Si—H, thereby making a density of the interface state lower. When a conventional nonvolatile semiconductor memory device is manufactured, the step of the hydrogen annealing is usually used.
However, it is becoming clear that even though the hydrogen annealing is used to an EEPROM that includes a tunnel oxide film (a gate oxide film), a worse characteristic thereof can not be avoided. A rewrite operation of the EEPROM is achieved by injecting electrons from a semiconductor substrate to a floating gate, or emitting electrons from a floating gate to a semiconductor substrate by FN tunneling effect or hot carrier injection effect. However, energy of Si—H bond in the tunnel oxide film is relatively low, and the Si—H bond in the tunnel oxide film can be cut due to repeating the rewrite operation, thereby resulting in a worse characteristic thereof.
On the other hand, instead of the hydrogen annealing, a variety of steps of manufacturing a semiconductor device with the deuterium annealing are proposed. For example, it is proposed that deuterium bonds (Si—H) are formed at an interface between a gate electrode and a gate oxide nitride layer, thereby making characteristics of an insulating break resistance better (See Japanese laid open koukai Hei 11-274489). It is also proposed that deuterium is injected at an interface between a gate insulating film and a silicon substrate, thereby preventing a damage of the gate insulating film (See Japanese laid open koukai Hei 11-284183). It is also proposed that, in a device that includes a transistor and a high dielectric capacitor, deuterium is injected at an interface between a gate insulating film and a silicon substrate, thereby making a threshold voltage of the transistor and a retention characteristics of the high dielectric capacitor better (See Japanese laid open koukai 2000-77621). And also, it is proposed that deuterium is injected after forming a gate insulating film that includes nitrogen or metal, thereby preventing an occurrence of electron traps that is caused by hot electrons (See Japanese laid open koukai 2002-118252).
Recently, a memory cell array in a NAND type EEPROM is covered with a silicon nitride layer (SiN) in order to form contact holes that are connected to a bit line. The silicon nitride layer (SiN) includes a lot of Si—H bonds and/or N—H bonds. When a thermal step is done with high temperature that is more than temperature at which the silicon nitride layer (SiN) is deposited, the Si—H bonds and the N—H bonds in the silicon nitride layer (SiN) can be cut, and atomic hydrogen is emitted from the silicon nitride layer (SiN). Thereby, the tunnel oxide film under the silicon nitride layer SiN is resolved by the atomic hydrogen emitted from the silicon nitride layer, and a number of Si—H bond in the tunnel oxide film increases. The Si—H bonds in the tunnel oxide film would cause a worse characteristic of the memory cell transistor, if the rewrite operation was done repeatedly.